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Fall 2012: Project 2: CNC

Background: A computer numerical control (CNC) machine is a machine that cuts/assembles material using computer automation. Once the designs are drawn in a computer aided drafting program like AutoCAD, the computer instructs the machine head where to move (x, y, z coordinates) and what to do when it gets there (speed, cutting action, etc.).

Design: The design for our CNC mill will consist of three parts: platform, electronics, and software. (The hardware design is based on the work of Tom McWire.)

Platform: The platform consists of two horizontal layers - one which moves in the x-direction and the other in the y-direction - and a motor mounted on an arm capable of moving vertically in the z-direction. Each layer moves on rails and is pulled by a stepper motor.From left to right, the base with arm, x-direction platform, y-direction platform, and z-direction platform with motor.

Electronics: The motor is an old food blender. The platforms will be moved using bi-polar stepper motors with threaded rods attached to their shafts. A L298N Dual H-Bridge will be used to power the motors with the Arduino board providing control. (The motor control circuit is based on the work of Lewis Loflin.)

Program: A complete CNC program will not be produced for the project. A simple CNC program will be written in Excel which will then convert the code into x, y, and z coordinates. The Arduino can read input through a serial port and then output the required motor controls. Stepper motors are controlled by alternating the polarity of the motor's windings. Using the circuit diagram above, the bi-polar step motor can be controlled in three ways by changing the polarity. If the polarity is changed in sequence going down, the motor turns one direction, and if the polarity is changed in sequence going up, the motor turns in the opposite direction. (More can be read about step motors at Stepperworld.)